1. Field of the Invention
This invention relates to an improved mineral-filled urethane pre-polymer-based surface coating comprising free isocyanate groups that incorporates thiophene and arylsulphonylisocyanate compounds. This improved composition delays or prevents cross-linking or gelation of the surface coating during storage.
2. Description of the Background
Surface coatings are known that are based on the cross-linking of urethane pre-polymers with free isocyanate groups. These surface coatings may be compounded to include mineral components which enhance the durability and binding of the urethane coatings. However, the inclusion of these minerals accelerates the cross-linking and gelation of the surface coatings during storage. This is particularly pronounced when silicate fillers are present. Urethane pre-polymers rapidly cross-link under storage conditions even in the absence of moisture. This cross-linking leads to a progressive increase in viscosity that culminates in the gelation of the surface coating. In addition, the catalytic trimerization of the free isocyanate groups also contributes to the instability of silicate filled urethane coatings as is known to those skilled in the art. (Doyle, E.N., "The Development and Use of Polyurethane Products", New York, page 77 et seq. (1971)).
Other methods are known to reduce the reactivity of pigments utilized in coating compositions. In one method, the pigment is pre-treated with a pure isocyanate in a solvent so that all reactive groups found in the pigment are pre-reacted with the isocyanate. This protective step is conducted before adding of the urethane pre-polymer comprising terminal isocyanate groups. This prior art method, however, requires the addition of a solvent and is technically complex. (Bienemann, R. A., et al., Off. Digest 32:273 (1960); Saunders, J. H., "Polyurethane Chemistry and Technology", New York 2:533 (1964)).
DE-A-1900513, DE-A-2030316, DE-C-1245590, US-A-4383070 and EP-A-0275893 disclose methods for stabilizing preparations containing urethane pre-polymers monoisocyanates and, optionally, reactive fillers which would otherwise cross-link in the presence of moisture.
Another known way of stabilizing surface coatings which contain urethane pre-polymers comprising terminal isocyanate groups is by dissolving the pre-polymers in dimethyl formamide and adding 0.01% to 2% by weight of an ammonium salt of an organic acid as stabilizer.
Another method of stabilizing urethane pre-polymers in the presence of silicate fillers requires that the free isocyanate content of the preparation be above approximately 2.4%. When the isocyanate content is kept at this amount the storage time is extended to at least 12 months at room temperature.
Moreover, many solvent-based formulations are ineffective, because the urethane polymers geld when the solvents are evaporated in high vacuum.
Accordingly, there is still a need for an effective formulation of a mineral-filled, urethane pre-polymer-based surface coating that avoid cross-linking or gelation of the pre-polymers prior to use and have prolonged storage periods.